Kimera-VIO Files

• Added the concept of PipelineModule that standarizes the VIO parts and makes "plug-and-play" parallelization possible: now frontend, backend, mesher, visualization, loop-closure detection and pose-graph optimization (and perhaps more modules in the future) are all instances of a PipelineModule which provides a standard thread-safe copy-free interface for message passing, removes duplicated code, increases overall speed of the pipeline and makes it trivial to add new threads with other functionality.
• The pipeline now has three callbacks for sensor updates: left/right image and IMU data. This makes the ROS wrapper extra thin, as it simply feeds these callbacks.
• Better testing infrastructure to make it easier to log things and display stats: checkout here (and navigate through the tabs to see info: we also have frontend stats displayed.

Breaking changes: - We now ask users to system-wide install glog and gflags:

sudo apt-get install libgflags-dev libgoogle-glog-dev